It is often necessary in manufacturing operations to accurately control the temperature of one or more parts of the manufacturing process. Usually such control is critical to the success of the operation. In some operations the temperature must be varied in a predetermined way during the manufacturing process. For example, commercial chemical manufacturing processes are often carried out in a "batch" process. In such a process, chemical reactants are placed in a reactor vessel and and subjected to varying temperatures and reagents to produce a resultant compound.
Often the temperature of either the reaction vessel or the chemical mixture within must be controlled to a precise predetermined temperature in order to insure that the chemical reaction takes place properly.
In conventional chemical process systems, in order to control the temperature of the reaction vessel, the vessel is surrounded by an outer jacket through which a heat transfer medium is circulated. The temperature of the heat transfer medium which determines the temperature of the vessel is, in turn, controlled by means of temperature control apparatus.
The temperature control apparatus typically pumps the heat transfer medium through the reactor jacket and responds to signals from temperature sensors which sense the temperature of the vessel jacket and the chemical reactant batch. In response to the sensor signals the temperature control system provides the vessel with a medium which either heats or cools the chemical batch to maintain its temperature at a specified point.
Generally, in a large manufacturing plant, central heating and refrigeration plants are used to provide heat and remove heat from a plurality of reaction vessels in the plant. These central plants provide heat transfer media at predetermined temperatures. Consequently, many prior art temperature control systems simply circulate the appropriate heat transfer medium through the reactor jacket to provide the necessary heating or cooling. Since these control systems were made to operate with existing heating and cooling plants they were most often custom built for each reactor vessel.
These latter temperature control systems had severe problems if accurate temperature control was necessary or if the chemical batch mixture had to be heated during one portion of the process and cooled during another portion of the process so that rapid switching between heating and cooling media was necessary. In this latter situation, the reactor vessel jacket had to be flushed during each switch from heating medium to cooling medium if the media were not compatible, as was often the case. Even with flushing between switches, cross-contamination usually occurred causing sludge and corrosion problems.
To overcome the flushing and contamination problems, temperature control systems have been developed which use a single recirculating heat transfer medium and heat or cool the medium by means of heat exchangers to control temperature.
Some of these known single-medium systems utilize a heat exchanger in the recirculating loop to heat the transfer medium and can be connected to the central cooling plant to cool the recirculating medium. Other conventional temperature control systems utilize two heat exchange units connected in parallel to perform the heating and cooling function. One heat exchanger is provided with a cooling medium from the central plant and the other heat exchanger is provided with heating medium from the central plant. Since there is no intermixing of the heating and cooling media, cross-contamination is eliminated.
These latter control systems also had an additional advantage in that they could be manufactured as modular, self-contained systems and thus the expense for providing such a system was reduced over the conventional custom-built systems.
However, even such temperature control systems with one heat transfer medium have problems. First, it was found that neither the direct connection of the loop to the central cooling plant nor the parallel connection of heat exchangers could control the recirculating medium temperature to an accuracy sufficient for certain processes. Secondly, in order to provide sufficient capacity to accurately control the temperature of a given size reactor vessel, single-medium temperature control systems which utilized parallel heat exchangers had to use heat exchangers which could handle the maximum heat load generated by the vessel. If the vessel was not generating the maximum heat load, conventional single-medium, parallel-heat exchanger temperature control systems wasted energy because there was no way to tap directly into the central heating or cooling plants to handle peak heat load situations.
Another problem with single-medium systems was that they could damage the reactor vessel. Many reactor vessels are lined with glass because of the corrosive nature of the reacting materials and the glass liner is subject to damage by large temperature differentials. Such differentials can occur when a single-medium system switches between a cooling mode and a heating mode.
Accordingly, it is an object of the present invention, to provide a temperature control system which can control the temperature of a manufacturing vessel to an accuracy sufficient for most manufacturing processes.
It is another object of the present invention to provide a temperature control system which can operate directly with central heating and cooling plants to provide increased capacity during peak heat loads.
It is still another object of the present invention to provide a temperature control system which can conserve energy by utilizing the cheapest heating or cooling method available.
It is yet another object of the present invention to provide a temperature control system which can protect manufacturing vessels from damage due to thermal stress.
It is a further object of the present invention to provide a temperature control system which is electronically-controlled.
It is yet another object of the present invention to provide a temperature control system having an explosion-proof control unit located in the vessel area and a remote electronic control unit located in a non-hazardous area which can operate the control unit by means of the control signals.
It is a still a further object of the present invention to provide a temperature control system which can be quickly and easily reprogrammed to accommodate different manufacturing processes.